This application proposes to continue research into the transcriptional mechanisms that control development of serotonin (5ht) neuron transmitter traits and 5ht-modulated behaviors. Altered 5ht signaling has been implicated in the pathogenesis of numerous neurological and psychiatric disorders such as autism, sudden infant death syndrome (SIDS), depression, and anxiety. Many of these disorders are neurodevelopmental in origin and risk for acquiring them is influenced by heritable susceptibility factors. Thus, the general concept that stimulates the research proposed here is that genetically driven variation in transcriptional programs governing 5ht neuron development contributes to the pathogenesis of certain neurological and psychiatric disorders by adversely altering 5ht system activity. The focus of the proposed research is the Pet-1 ETS factor whose expression is initiated specifically in postmitotic 5ht neuron precursors before the appearance of 5ht and then maintained in all adult 5ht neurons. Loss of functions studies in the mouse demonstrated that Pet-1 is a critical transcriptional determinant of 5ht neuron phenotype and 5ht-modulated behaviors. These findings support the existence of a Pet-1 dependent transcriptional program that impacts 5ht- modulated behaviors through its control of embryonic 5ht neuron development. The new aims proposed here will address the following questions: 1) Is the FEV ETS factor the functional human orthologue of Pet-1 and does a FEV-dependent genetic program govern human 5ht neuron development? 2) Does the level of Pet- 1/FEV activity determine the level of 5ht neuron transmitter traits and 5ht neuron function? 3) What role does Pet-1 play in mature 5ht neurons? 4) Is the transcriptional control of Pet-1 and FEV conserved? The proposed aims will be investigated with molecular genetic approaches and will be greatly facilitated by our newly developed genetic based tools designed to access 5ht neurons, in vivo. If FEV functions in human 5ht neuron development then alterations in FEV activity brought about either by genetic or environmental factors are a potential source of risk for psychiatric or neurological disease and interindividual variation in behavior. The proposed studies are likely to have a long-term impact on molecular psychiatry research by advancing an understanding of the genetic mechanisms that govern 5ht neuron development and determining how these mechanisms impact behavior and physiology.